1. Field of the Invention
The present invention relates to an optical disk drive for performing at least one of writing and reading of signals using near-field light, an optical disk apparatus mounted with the drive, and a method for driving the apparatus.
2. Description of the Related Art
To increase the recording density of an optical disk on which data is written or read using laser light, an optical disk apparatus for writing or reading signals using near-field light has recently been proposed. According to a proposed technique for the optical disk apparatus utilizing near-field light, a gap between an optical disk and the end surface of a solid immersion lens (SIL) disposed in a light focusing element, such as an objective lens unit, is controlled to be a distance (near field) where near-field light is generated. The distance is generally half the wavelength of input laser light. For example, in the use of blue-violet laser light with a 400-nm wavelength, the distance is approximately 200 nm.
Upon starting control of the gap, an overshoot of 1 μm or less may occur. In a far-field optical system in an optical disk apparatus for writing or reading signals on a compact disk (CD) or a digital versatile disk (DVD), the overshoot is insignificant. In an optical recording and playback apparatus using near-field light, the overshoot is a serious problem. In other words, if the overshoot of 1 μm or less occurs upon starting the control, the SIL collides with the disk, resulting in damages of them.
One approach to solving the above-mentioned problem uses a technique for controlling the gap on the basis of the amount of returning laser light reflected from the disk. For instance, in the case of using laser light of a 400-nm wavelength, when the gap length is generally half the wavelength or less, a near-field state is produced. Accordingly, assuming that the gap is more than 200 nm, i.e., in a far-field state, when light emitted from a laser source is incident on the end surface of the SIL at an angle at which total reflection occurs, the light is totally reflected by the end surface of the SIL, so that the amount of returning light is constant. When the gap length is 200 nm or less, i.e., in the near-field state, light incident on the end surface of the SIL at the angle of total reflection partially passes through the end surface thereof, so that the amount of returning light is reduced. When the gap between the SIL and the disk is zero, i.e., when the SIL is in contact with the disk, the entire light incident on the end surface of the SIL at the angle of total reflection passes through the end surface thereof, so that the amount of returning light is zero. In this technique, the amount of returning light is detected by a photodetector and data indicating the detected amount is fed back to an actuator (e.g., a two-axis device for performing focusing servo and tracking servo operations) for the SIL, whereby a gap servo operation is performed. This approach is disclosed in, e.g., U.S. Pat. No. 6,701,913 (Patent Document 1).
Another approach utilizes a technique for setting a threshold used to determine, e.g., whether the near-field state is produced, approaching the SIL to the disk until the threshold is detected, adding a servo voltage to an approach voltage after the threshold is detected, and then performing the gap servo operation. This approach is disclosed in, e.g., Japanese Unexamined Patent Application Publication No. 2004-30821 (Patent Document 2). In this case, the approach voltage is a ramped voltage (see FIG. 8 of Patent Document 2). Since the SIL is moved at the initial velocity upon starting to approach to the disk, the SIL fluctuates at the start of the approach (refer to FIG. 12 of Patent Document 2). After that, the SIL is moved in accordance with the ramped voltage that is reduced to a target value (corresponding to a target gap of several tens of nm).